I wasn't expecting that at all If everything is securely in place and it won't move, then I guess it's OK. If the brown empty pcb is intented for the preamp, then you might have some mains hum when it gets amplified through it. Keep the input signal as far away as possible from that psu.+1 for creativity

I wasn't expecting that at all If everything is securely in place and it won't move, then I guess it's OK. If the brown empty pcb is intented for the preamp, then you might have some mains hum when it gets amplified through it. Keep the input signal as far away as possible from that psu.+1 for creativity

Yes, that's where the preamp will go. I'll move the input jack farther away from the PSU and the PCB input will sit right underneath it, so the wire will be a few millimeter long, like 12 mm or so. Hope that works just fine. I guess I should have mounted the PSU with the mains facing the opposite side. Will fix that. Man, I appreciate your input, really do, Thank you!

I wasn't expecting that at all If everything is securely in place and it won't move, then I guess it's OK. If the brown empty pcb is intented for the preamp, then you might have some mains hum when it gets amplified through it. Keep the input signal as far away as possible from that psu.+1 for creativity

Yes, that's where the preamp will go. I'll move the input jack farther away from the PSU and the PCB input will sit right underneath it, so the wire will be a few millimeter long, like 12 mm or so. Hope that works just fine. I guess I should have mounted the PSU with the mains facing the opposite side. Will fix that. Man, I appreciate your input, really do, Thank you!

yes I guess any low signal circuit could be termed a preamp but a lot of them will colour the sound in a negative way and because it's hard wired into the Amp you are stuck with one sound only.I'm yet to meet a guitar player who is happy with all 2/3 channels of his Amp.

What you need is a 2 or 3 stage preamp the first matching the guitar with some gain and the second stage runs a tone control.If the tone is passive then you have to adjust the gain to pull up the signal loss in stage 2.

This can be done with one dual opamp running single or dual supply.Those fancy circuits with a whole pile of parts is just going to be a nightmare for the novice. (hint)BTW, I have tested that vulcan circuit and it's not that great,, as a pedal effect it may help but there are likely better (easier) ways for simulating the valve sound.

Be aware of sound clips as they can be misleading,, a lot of demos are just $20 distortion circuits (that have been around for years) plugged into Valve amplifiers that most of us can't afford to even look at. They hit the pedal and your hear a sound to die for.

So you go buy that pedal and plug into a small budget bedroom SS amp and it sounds nothing like the demo,, most will feel ripped off.

Reality; Almost any crap dirt pedal will sound stunning through a big monster Valve Amp,, But not quite so easy to do with all SS rigs,, though some Can do that trick.

Reason;With any half decent Valve amp you only have to send it a bigger signal from any old dirt pedal and the Valves do the magic because of the way they work.(A whole other subject that has filled many books)

This magic trick will never happen with a small simple design bedroom SS Amp combo with a 6 inch speaker.

So with All SS circuitry you need to do a lot of fancy circuit tricks to get same or similar outcomes. A SS power stage is very different to a Valve power stage and most SS Pwr stages won't compress like a Valve rig so you have to do most of the tricks in the pre stages.

MY advice is build a basic preamp, a single clean channel is good because you have a basic clean sound which is not too coloured. You then try out different pedal circuits which work with your Amp.

I have a simple Casino bedroom Amp here (read Cheap) and with a few tweaks itactually sounds ok through a small speaker but much better through my quad box.

The added dirt circuit is not brilliant but the clean is good.I've tweaked the tone shaping to wipe off the excess hi freq that these small amps produce which gives a much smoother sound but don't be fooled there is more treble than you will ever need ,,unless you have tin ears.

hope it helps,, Phil.

Phil, if you read this, can you please let me know which chips are those in your Casino 12? I searched the forums to no avail

I ordered LM386 & TL082's a couple of weeks ago, will any of those do?

EDIT: I just reread your post and you mentioned that any dual opamp would do, so I'm guessing the TL082 is fine?

Yes any of the usual opamps will do the same job give or take an electron but TL07X is likely a better chip than lm4558.

The TL082 has low current draw which might be good for battery powered circuits but that is not needed in a wall powered unit.But yes use those if you wish.

The LM833 is aimed at audio and is a top shelf chip. I doubt you will note much difference in performance with any of the usual chips.Heck it's a guitar amp so the last thing you want is 20/20 bandwidth.

The LM386 is powerchip and not really much use in the preamp unless you want it for some special booster. I can't remember the name but a couple of pedals use those to get big distortion sounds.Others here may recall those circuits??Phil.

Yes any of the usual opamps will do the same job give or take an electron but TL07X is likely a better chip than lm4558.

The TL082 has low current draw which might be good for battery powered circuits but that is not needed in a wall powered unit.But yes use those if you wish.

The LM833 is aimed at audio and is a top shelf chip. I doubt you will note much difference in performance with any of the usual chips.Heck it's a guitar amp so the last thing you want is 20/20 bandwidth.

The LM386 is powerchip and not really much use in the preamp unless you want it for some special booster. I can't remember the name but a couple of pedals use those to get big distortion sounds.Others here may recall those circuits??Phil.

Thanks Phil. I've retaken the project this weekend and been doing some reading about JFET preamps, just because I have some of those, but it looks like JFETS either don't provide enough gain to drive a preamp with a full tone stack or if they do, they clip too easily (J201) to get a clean channel as you suggested, so I'm going with your Casino 12 (just the clean channel for now). I don't have trim pots to bias those JFETS anyway and for what I could gather they tend to be all over the place in terms of their electrical properties so I can't just go by an schematic and expect it to work right off the bat.

And finally, I have it wired so that the preamp output is fed to both channels of the power amp even though I'll only be using one speaker most of the times. This is a bad idea, isn't it? That will halve the power amp input impedance (from 30K Ohms to 15K Ohms) and according to Duncan's tone stack calculator, will attenuate the signal another 6dB (shouldn't it be 3dB?). So if that's correct, does it even make any sense to add a switch to connect the second channel to use a second speaker, considering that doubling the power output (+3dB) and doubling the cone area (another 3dB) will simply cancel out the 6dB loss from the input impedance halving? I don't think my 12V 1.5A wall wart power supply can cope with both channels anyway (some 6W per channel at 12V according to the TDA7297 datasheet)

Maybe I can use that second channel for a switchable headphone jack output by connecting 50 - 100 Ohms / 2W resistor in series to attenuate the signal enough?

Oh I just realized that you obviously have a single 12 volt supply,, in which case you will need to alter the Casino circuit as it assumes split supply.You will need a bias voltage, look at pedal circuits for clues.If you can't work it out I'll redraw the schematic for single supply.

Re the 2nd half of the power chip,, just leave the pins open as it will just idle with no load and not draw much power from the supply.with some chips you can just ground the input to be sure nothing weird happens.Phil.

Oh I just realized that you obviously have a single 12 volt supply,, in which case you will need to alter the Casino circuit as it assumes split supply.You will need a bias voltage, look at pedal circuits for clues.If you can't work it out I'll redraw the schematic for single supply.

Re the 2nd half of the power chip,, just leave the pins open as it will just idle with no load and not draw much power from the supply.with some chips you can just ground the input to be sure nothing weird happens.Phil.

Re: bias voltage, I guess that would be a voltage divider to get +6V, GND, -6V? Perhaps with large resistors to minimize current through the divider?

This is where you can resort to a simple rule of thumb; which is the 1k per volt rule.So to get a bias voltage with a 12volt supply you need at least 2x 6k resistors so 10k x10k is fine and won't load down the supply,,in fact you can go larger even up to 100k as it makes little difference.All you want is a reference voltage so the opamp can work at that offset, a vitual ground (the bias voltage).

So R2 500k now goes to bias voltage not ground.Just look at the many dirt pedal circuits on the net and you will see how that is done.

I'm busy right now but I'll try to redraw it up in the next day or so.Phil.

R2 can be anywhere from 220k up to 1Meg.The input Z will always be a lower number than the value of R2,, but not much.The higher Z you go the more sensitive the input but high Z can be problematic if you have lots of hi gain pedals.

Try a few values and see/hear for yourself,, most will pick 1Meg because you get a bit more perceived volume but at the expense of early distortion in the power amp.As your power chip is small and only running 12 volts then there is not much headroom.Phil.

I got some TDA7267A's in the mail yesterday (the chip I initially planned to use) so I thought I would do something with them and take the opportunity to practice with a little 1W amp. I used an MPF102 JFET for the preamp (Feltzer valve), in preparation for the drive stage of the larger project but I don't like too much the way it sounds overdriven. Probably to be expected considering I'm using some cheap 3'' speakers.

There's something I don't understand about this little amp: it works like a charm on a 9V or 12V wall wart, but when powered by a 9V battery it sounds like crap, and I get these weird oscillations that sound like heli rotors. I measured 150mA at the battery, doesn't sound like too much of a current draw to choke the battery does it?